For many years the exclusive design of intersection weld type fittings was characterized by a tapered skirt which lends itself quite readily to forging. The main body of the fitting in the longitudinal plane has a wall thickness of between 4.0 to 7 times the nominal wall thickness of the main pipe. Although full replacement of area was often achieved for the purpose of compliance with the pressure piping codes, the very great difference in wall thickness between the fitting and the main pipe leads to severe stress concentration factors in the fatigue environment. As a result stress intensification factors of 1.75 and higher are often found in forged fittings. Additionally, the heavy wall thickness of the fitting requires a substantial amount of weld for proper installation. A typical forged fitting of this character is disclosed in Durham U.S. Pat. No. 1,966,403 except for the fact that the wall thickness of the fitting as shown in the drawings is too small in relation to the main pipe to provide adequate burst strength.
While a cylindrical single bevel fitting is superior it still requires a substantial weld metal volume which is a problem which increases with the increase of fitting size.
Many of the problems encountered with the above discussed fittings were solved in the fitting disclosed in Best U.S. Pat. No. 3,894,757 which teaches the employment of a cylindrical branch pipe connection with a weld bevel and a transition bevel of uniform width around the contour of the fitting. While a decided improvement, a typical problem with this arrangement is with high pressure piping subject to external bending moments where due to the notch effect in the longitudinal plane of the intersection weld, a high stress intensification factor is experienced which leads to premature failure due to metal fatigue. The present invention reduces very substantially the amount of weld required for the installation of the weld fitting as contrasted to the single bevel fitting, while it does not reduce the fatigue resistance of the branch connection or the internal pressure performance achieved by the single bevel fitting.
In accordance with this invention a marked improvement over the fittings of the prior art is achieved by employing a cylindrical fitting having a weld bevel and a transitional bevel extending from the center of each ear towards the centers of the crotches with the width of the transitional bevels gradually decreasing. This provides for economic welding of the fitting to the main pipe and provides superior fatigue resistance by overcoming the notch effect of previous cylindrical fittings.